In radiological practice, it is common to display different exposures pertaining to a patient study in a predefined format. This feature is known as a hanging protocol. In film-based operation, it means that the radiologist or operator hangs the films on the light box in a specific spatial arrangement according to standard or local preferences. Determination of the orientation or reflection of an examination type, or verification of it when it is available, is beneficial to the correct display of many examination types.
For example, thorax examinations are always hung in such a way as to display the ribcage in the upstanding direction and showing the heart shadow at the right side with respect to the centerline. However, for reasons of exposure and read-out geometry, 90-degree orientation changes may occur in digitally acquired images.
In particular for chest images acquired with a computed radiography modality, guaranteeing a correct orientation is a major problem, because the cassette may be wrongly oriented at exposure time due to patient conditions and clinical requirements. Orientations different from the standard upright orientation are reported to be as high as 40%. Especially in high throughput hospital departments, radiologist and clinicians perceive the task of rotating the image to the standard orientation as a cumbersome task and a waste of time. Moreover, when automatic diagnostic data processing has to be performed on the digital image, a standard orientation must be available for correct operation of the CAD (computer-aided diagnosis) algorithms or subsequent operations such as annotations, measurements and archiving.
A similar problem exists in screening mammography, where standard 4-views of left and right breast are taken for each of the incidence directions (cranio-caudal (CC) and medio-lateral oblique (MLO)). These views are typically displayed in a mirrored fashion, such that the thorax edges or both breasts are central and touching, and the left breast image being displayed on the right and the right breast image being displayed on the left. However, because both breast images are acquired in a similar manner, and it is in general not known which image is either corresponding to the left or the right breast, one image must be flipped before it can be positioned adjacent to the other image. In conventional screen-film imaging, X-ray opaque lead letters are radiographed simultaneously (RCC, LCC, RMLO and LMLO), and the RCC resp. RMLO films are flipped manually prior to hanging them on the right of the LCC resp. LMLO on the light box.
Digitally acquired mammograms may still be read in a conventional way by printing them on film and displaying them on a light box. Pairs of mammograms (e.g. the RCC/LCC pair and the RMLO/LMLO pair) may be printed on a single large film sheet or on two smaller sized sheets. Generally, the print margin of a hardcopy machine is adjustable, so as to minimize the non-printed portion of an image. For mammography hardcopy, the print margin corresponding to the thorax side is kept as small as possible, so that a right-left pair of images, when viewed simultaneous and in close vicinity, shows a minimal non-diagnostic viewing area in between both images. Therefore, when using a pair of small sheets to print left and right images respectively, means to identify the thorax side automatically prior to printing them is needed, because the thorax side position is generally not known or it may not be assumed to be known. Likewise, in the large film option, where both images are printed on one file sheet in order to compose the image such that the right image is touching in a mirroring manner to the left image, knowledge of the thorax side of both images is needed as well.
Digital mammography may be read on a computer display or viewing station without having resort to printed mammograms, a viewing condition known as softcopy reading. However, also here, the sub-examination types identifying right and left images may not be known at display time. Furthermore, the thorax edge orientation may not be standardized, e.g. it may either touch the left or right or the upper or lower image border. Hence, there is a need to accomplish the mirrored viewing disposition in an automated way.
Generally, a hanging protocol function enables users to arrange and display images on medical viewing stations to suit specific viewing preferences. To this purpose, the sub-examination is used to assign the sub-images pertaining to a patient study of a body part to a position in a preferred display layout of that examination type. When the image sub-type is known, hence its position in the layout is determined, the image can still by oriented in 8 different ways: it can be oriented correctly, or it can be rotated of 90, 180 or 270 degrees; in any of these four cases, the image can also be flipped around a vertical (or horizontal axis). Therefore, there is a need to derive the orientation of the image automatically, to assure viewing according to the radiological standard or local preferences.
The problem of orientation detection and occasional correction therefore consists in detecting the orientation as one of 0, 90, 180 or 270 degrees with respect to an image coordinate axis. When the orientation equals 0 degrees, no orientation correction needs to be performed, otherwise the image must be anti-rotated to have it in the upright direction (head-neck region on top of the corrected image).
The orientation detection solution must further be insensitive to a number of disturbing image characteristics such as                the presence of collimation borders        rotation of the centerline of the thorax with respect to the vertical axis        the presence of other non-thorax regions, such as head/neck, sub-diaphragm region and abdomen region        unilateral hyper-dense appearance of a diseased lung region        